Selected positions of acyl chains are affected differently by antibody binding which results in decreased membrane fluidity.

We have studied the interaction between monoclonal anti-trinitrophenyl antibodies (IgGl and IgG2a) and haptenated phospholipid vesicles using stopped-flow fluorometry. Conformational changes of the antibodies were induced very rapidly (within 0.1 s) after binding to lipid haptens (TNP-Cap-DPPE) on the membrane surfaces. Conversely, after that, the bound antibody molecules decreased the degree of molecular motion at different depths in the bilayer, ranging from the polar head group to the terminal methyl groups of the fatty acyl chains. Such an effect reaches all places of the bilayer within 40 s at 25 degrees C.